Summary of Work: Epidemiologic studies have suggested that exposure to electromagnetic fields (EMF) may cause a slight increase in the incidence of some forms of cancer; however the mechanism responsible for this is unclear. Electromagnetic field exposure is reported to decrease circulating melatonin levels, and melatonin is reported to be oncostatic. This project's goals are to elucidate the signaling pathways involved in the oncostatic action of melatonin. We have confirmed that BG1 cell numbers are significantly reduced when cells are grown in the presence of melatonin. We have determined the dose response relationship between melatonin and reduced cell numbers. We investigated whether melatonin signaling involves an alteration in cytosolic free calcium (Cai), and found that acute addition of melatonin does not alter Cai. We are investigating whether melatonin is acting via receptor mediated or non- specific pathways (e.g. as an antioxidant). Melatonin has been reported to bind to plasma membrane receptors, blocked by luzindol, as well as tan orphan receptor of the nuclear receptor superfamily. Growth inhibitory effects of melatonin are not blocked by luzindol, suggesting they are not mediated by plasma membrane receptors. Addition of CGP52608, a nuclear receptor agonist, resulted in reduced cell counts at 10-8 and 10-7M, suggesting a role for the nuclear receptor. Reduced cell numbers observed with cells grown in the presence of either melatonin or CGP52608 could be due to either inhibiting or slowing cell proliferation or alternatively due to increasing cell death. Cells grown in the presence of CGP52608 showed slight DNA fragmentation as measured by agarose gel electrophoresis. Future studies will investigate the following: 1) We will use the fluorescence activated cell sorter analysis and 3H thymidine incorporation to determine whether melatonin causes a decrease in cell proliferation? 2) We will use DNA gel electrophoresis and TUNEL to determine if melatonin induces apoptosis. Trypan blue incorporation will be used to determine cell death by either necrosis or apoptosis.